1. Field of the Invention
The present invention relates to a nondirectional acoustic generator and speaker system wherein two acoustic generator units or speaker units are disposed such that diaphragms thereof become as near as possible and driven in phase to thereby radiate sound waves suitable for obtaining a particular directivity or nondirectivity.
2. Related Background Art
There are known various types of conventional nondirectional speakers. For example, a speaker shown in FIG. 23 has a plurality of speaker units 22 mounted nondirectionally on a cabinet 21 to perform a respiratory operation, a speaker shown in FIG. 24 uses a piezoelectric film 23 formed in a conical or truncated conical shape to make the diaphragm itself nondirectional, and a speaker shown in FIG. 25 uses reflectors 24.
There has been proposed also a speaker shown in FIG. 26 wherein drivers (speaker units 22) for reproducing low frequency components are slanted upward, and high frequency components are adapted to be reflected by a spherical body 25 (e.g., Japanese Patent Publication No. 62-221299).
A conventional horn type speaker 31 shown in FIG. 27 on the other hand is constructed of a driver unit 33 which is driven by energy from a magnetic circuit upon application of a signal to a voice coil mounted on a diaphragm 32 formed in a horn shape, a throat unit 34 for equalizing the phase of high frequency components by means of a phase equalizer 34a, and a horn section for matching acoustic impedance.
In a conventional speaker system for reproducing a low frequency sound, as shown in FIG. 28 a speaker 43 is mounted on a partition plate 42 fixed within a cabinet 41, the volume ratio of air chambers 44 and 45 partitioned with the partition plate 42 is set at 1:1.2, and an acoustic filter formed by the air chamber 44 and a port 46 attenuates high frequency components. A 3D (three-dimensional) system having an improved version of the above-described driver unit is shown in FIG. 29 wherein there are provided two speakers 47 and 48, and a bathlet type acoustic filter which is formed by providing another port 49 in an air chamber 49 in addition to a port 46 in an air chamber 46. The speakers 47 and 48 are inputted with right and left stereo signals to realize a 3D speaker.
The nondirectional speaker shown in FIG. 23 is associated with some disadvantage that the baffle mounted with a plurality of speaker units 22 is difficult to be worked, resulting in high cost and a limited shape of speaker mounting frame.
The speaker made of a piezoelectric film shown in FIG. 24 has disadvantages of a necessity of impedance matching, low efficiency, and insufficient amplitude.
In the case of the speaker shown in FIG. 25 which is made nondirectional in the horizontal or vertical direction by the provision of the reflectors 24, there also arises a problem that the directivity and frequency characteristics depend on the shape of the reflector.
The speaker shown in FIG. 26 has an uneven frequency characteristic of reflected sounds by the spherical body 24 so that the directivity shows a beam shape which cannot be eliminated. In addition, there is a problem that the low frequency sound regenerating portion is theoretically far from a simple sound source. Further, in mounting the driver and the spherical or semi-spherical body for reflecting high frequency sounds, there is some mismatching between the size of the magnetic circuit and the input terminals of the drivers, resulting in a large size of the actual spherical body.
The directivity characteristic of sound waves for the conventional horn type speaker shown in FIG. 27 depends on the shape of the horn, covering only 90 to 140 degrees at most.
The speaker shown in FIG. 26 can be considered to have a partial space operating as a horn. The reason for this is that dome type speakers are driven in phase to radiate and diffuse sound waves in the direction toward a space defined by a spherical or partially spherical body so that the space defined by the wall surface of the spherical body operates mainly to reflect and diffuse sound waves. However, the speaker shown in FIG. 26 has a disadvantage that the efficiency of acoustic load and the frequency response characteristic to be obtained by such a horn is not so good.
Next, the speaker system shown in FIG. 28 requires a large air chamber. The 3D system shown in FIG. 29, although it is dedicated to low frequency reproduction only, radiates high frequency components from the port so that it has a particular directivity. Further, the conditions of the air chambers are not uniform near the diaphragms thereby increasing vibrations of the cabinet. Furthermore, the acoustic filter is defined only by the air chamber 45 and port 49 so that attenuation is small at lower frequency.